How Do I Know If I'm Depressed?

I always feel like crying and sometimes just break out in tears. The slightest thing makes me cry. I am easily irritated and generally feel nervous, worried, there's always something on my mind. I have an extremely difficult time reading, staying focused. I am very sluggish, it takes me a long time and a lot of effort to do everyday things. I feel very alone and sometimes I feel worthless, like no one cares about me. I have only cut once but it wasn't bad at all and it was for a stupid reason, I was doing math and I was over whelmed and crying because I didn't understand it. I don't know if Im depressed or just too emotional, I don't know. Do you think Im depressed? Please don't be rude. Thanks.